Can anyone recommend me any manga with really good art that could help me with my drawing?...

Those are very colorful and beautiful. I hope you'll like them.
http://www.mangago.zone/read-manga/19_days/
http://www.mangago.zone/read-manga/the_specific_heat_capacity_of_love/
http://www.mangago.zone/read-manga/mosspaca_advertising_department/
http://www.mangago.zone/read-manga/tamen_di_gushi/
http://www.mangago.zone/read-manga/noblesse/

Those are not colored, but the art is gorgeous:
http://www.mangago.zone/read-manga/kuroshitsuji/
http://www.mangago.zone/read-manga/shinrei_tantei_yakumo/
http://www.mangago.zone/read-manga/ilegenes/
http://www.mangago.zone/read-manga/karneval/
http://www.mangago.zone/read-manga/makai_ouji_devils_and_realist/
http://www.mangago.zone/read-manga/vassalord/

I think takarai rihito's art is good, like 'Hana no Mizo Shiru' and 'Seven days'

I really like one piece & fairy tail drawing. They have details. They don't even use coloring. Just pen rendering but the outcome are great. They have different angle & perspective of view. Complete background. I learned my perspective drawing and many things from one piece. It helped me increased my rendering skill

hello i'm a finearts graduate and i draw so for now, i recommend that you try to avoid mangas with hyper realism yet (like onepiece, fairytail, happy yarou saga, yamane-sensei and any clamp drawings... i'm sorry. just go back at it later on) since it will make a mess in your foundation and pace on the long run. look for a more basic approach in drawing. noblesse is nice but the artist has some lazy moments so i don't recommend. basically, what i want to say is steer away from any webtoons for now because those are usually done in a short timeframe that they are mostly unpolished.

Manga's that can be used for studying (bg, foreshortening and character designs)
- ao no exorcist
- naruto (best BG)
- http://www.mangago.zone/read-manga/ruler_of_the_land/ (great detailing)
http://www.mangago.zone/read-manga/ameiro_paradox/ - isaku sensei's illustration are simple yet on-point. great for reference.
- http://www.mangago.zone/read-manga/kozure_ookami_inoue_satou/ (check satou-sensei's human drawing of varying ages)